Nowadays, in the mainstream of integrated circuit production, a power semiconductor device withstanding a high voltage or high current and an analog or digital circuit operating at a low voltage are integrated into the same integrated circuit chip. The integration of the power semiconductor device and the analog or digital circuit will gradually replace the conventional distributed circuit because the distributed circuit is bulky and costly.
For example, a lateral diffused MOSFET transistor (also referred as a LDMOS transistor) is a metal oxide semiconductor device capable of withstanding a high voltage. In the LDMOS transistor, a channel region is laterally extended in a direction parallel with a surface of a substrate. FIG. 1 schematically illustrates an equivalent circuit of a lateral diffused MOSFET transistor. As shown in FIG. 1, both ends of an equivalent diode 10 are respectively connected to the source terminal and the drain terminal of the lateral diffused MOSFET transistor. That is, the equivalent diode 10 denotes the breakdown voltage of the lateral diffused MOSFET transistor.
In the practical applications, the integrated circuit chip is usually connected with an external inductive load according to the circuitry requirement. During operations of the circuitry, the diode 10 is readily suffered from an unexpected high voltage. If the unexpected high voltage exceeds the rated operating voltage of the lateral diffused MOSFET transistor, an over-current may burn out the lateral diffused MOSFET transistor. Therefore, there is a need of providing an improved high voltage semiconductor device so as to obviate the above drawbacks.